The present invention relates to a light receiving element using a GaN group semiconductor material.
To cope with the increasing density of integrated circuits, a stepper (step-and-repeat photolithographic system with demagnification) for forming a fine circuit pattern therefor is required to be capable of drawing more finely at higher resolution. Therefore, the laser beam used for exposure has been changed to that having a shorter wavelength range, from blue light to ultraviolet light, and changing the light having a wavelength of 248 nm (KrF excimer laser device) currently in use to the light having a wavelength of 193 nm (ArF excimer laser device) has been contemplated.
During the exposure step by the above-mentioned stepper, a part of the laser beam is received by a light receiving element and changes in the output and the like are monitored. As the light receiving element, a photodiode (PD) is useful. PD includes those made from Si group semiconductor materials. When the laser beam has a powerful energy, like the above-mentioned light at 248 nm, an Si group PD shows dramatic degradation, thus necessitating frequent replacement for a new one.
PD includes those based on many light receiving principles, one being a Schottky barrier type PD. With regard to the Schottky barrier type PD, wherein ultraviolet light is the receiving target, the invention disclosed in JP-A-61-91977 can be mentioned. This PD has a constitution wherein an AlGaN layer is grown on a sapphire substrate via an AlN buffer layer, and on this AlGaN layer are formed a Schottky electrode (electrode connected to form a Schottky barrier) and an ohmic electrode.
The PD of the above-mentioned publication has a structure wherein, as shown in FIG. 4(b), a light L3 (light receiving target) that enters from the substrate side reaches an area 210b right under a Schottky electrode 220 (a part of depletion layer spreading on the semiconductor side), where it is received. An ohmic electrode 230 is also formed on the upper surface 210a of a semiconductor layer 210. It is evident from the structure, wherein the Schottky electrode occupies a large area of the light receiving surface and the light can enter only from the substrate side, that this PD allows light to enter from the substrate side. Also, this publication clearly states, xe2x80x9cWhen a photon enters a depletion region under the Schottky barrier through a transparent Al2O3 substrate, a pair of electron-holes is produced.xe2x80x9d
However, the PD of the above-mentioned publication has the following problems.
{circle around (1)} Due to the constitution wherein the light to be received is passed through a thick AlGaN layer from a sapphire substrate, and led to the back of the Schottky electrode where it is received, the light is absorbed by the AlGaN layer and the sensitivity becomes low. In particular, a shorter wavelength of the ultraviolet light to be received is associated with a greater energy of the light, and as it separates from the band gap of AlGaN, an absorption coefficient of the light in the AlGaN layer suddenly becomes greater, and the light may not be able to reach a depletion layer region, which is formed by a Schottky junction, or a vicinity thereof at all.
{circle around (2)} For the above-mentioned reason {circle around (1)}, the wavelength range of a detectable light is limited to a narrow range near the band gap of AlGaN. That is, a sensible wavelength range is narrow.
PD also includes those of a photoconductive type. A photoconductive type PD is a light receiving element wherein a current is taken out by utilizing a phenomenon (photoconductive effect) that the conductivity of the crystals changes due to the carrier generated in a light receiving layer (generally, a semiconductor crystal layer made to be a high-resistance layer) by light excitation, based on which the receipt of the light is detected.
A conventional photoconductive element, as shown in FIG. 8, has a constitution wherein both positive and negative ohmic electrodes 120, 130 are disposed facing each other on the surface of the light receiving layer 110 as a light receiving surface. The light L4 is capable of exciting a semiconductor crystal layer 110 and generates a carrier. Due to the generation of the carrier, the conductivity between the electrodes varies. By such constitution wherein the voltage is applied between the both electrodes 120 and 130, the entry of light can be detected as a change in the current.
According to the above-mentioned structure of a photoconductive element, the generated carrier moves between the electrodes along the light receiving surface. The present inventors have found it problematic.
That is, the light receiving surface is literally a surface of or an interface between materials forming the light receiving layer, and is constantly exposed to a light having a severe energy. As a result, the light receiving surface and the surface layer thereof are subject to various quality problems, such as contamination from the surrounding environment during practical use, degradation of the semiconductor surface due to the incident light and the like, which are caused by the light receiving surface being an interface. With the structure of a conventional element wherein the carrier moves on the surface layer along the light receiving surface, therefore, the recombination velocity of the carrier dramatically changes, lowering the reproducibility of the detection results, and impairing the reliability of a photodetecting element.
It is therefore an object of the present invention to provide a light receiving element having superior resistance also to a light having a wavelength in the ultraviolet range.
Another object of the present invention is to provide a Schottky barrier type light receiving element having, besides the superior resistance also to a light having a wavelength in the ultraviolet range, much superior sensitivity as compared to conventional ones, based on a new constitution.
A yet another object of the present invention is to provide a photoconductive type light receiving element having a new structure capable of decreasing the contamination of and a degrading influence on a light receiving surface, besides the superior resistance also to a light having a wavelength in the ultraviolet range.
The light receiving element of the present invention is characterized by the following.
(1) A semiconductor light receiving element comprising a light receiving layer comprising a GaN group semiconductor and an electrode formed on one surface of the light receiving layer as a light receiving surface, in such a way that the light can enter the light receiving layer.
(2) The light receiving element of the above-mentioned (1), wherein the light receiving element is a Schottky barrier type light receiving element, the above-mentioned light receiving layer is a first conductivity type layer, the above-mentioned electrode formed on the above-mentioned light receiving surface comprises at least a Schottky electrode, and a total of boundary lines between areas of the light receiving surface covered with the Schottky electrode and exposed areas is longer than the length of the outer periphery of the light receiving surface.
(3) The light receiving element of the above-mentioned (2), wherein the above-mentioned Schottky electrode has a wiring pattern formed by strip conductors in combination.
(4) The light receiving element of the above-mentioned (2), wherein the above-mentioned strip conductors have a width of 0.1 xcexcm-2000 xcexcm.
(5) The light receiving element of the above-mentioned (2), wherein the above-mentioned wiring pattern is a comblike pattern.
(6) The light receiving element of the above-mentioned (2), wherein the above-mentioned light receiving layer is an uppermost layer of a laminate comprising one or more layers comprising a first conductivity type GaN group semiconductor formed on a crystal substrate, which element comprising an ohmic electrode formed on a layer other than the light receiving layer.
(7) The light receiving element of the above-mentioned (6), wherein the crystal substrate is made from a conductive material and the ohmic electrode is formed on the crystal substrate.
(8) The light receiving element of the above-mentioned (1), wherein the light receiving element is a photoconductive type light receiving element, the above-mentioned light receiving layer is a first conductivity type i layer, and the above-mentioned electrode formed on the above-mentioned light receiving surface is an ohmic electrode of one polarity, which element comprising an ohmic electrode of the other polarity formed on the other surface of the light receiving layer directly or via a first conductivity type and low resistance GaN group semiconductor layer.
(9) The light receiving element of the above-mentioned (8), wherein the ohmic electrode of one polarity is formed as a transparent electrode to permit an entry of light.
(10) The light receiving element of the above-mentioned (8), wherein the ohmic electrode of one polarity is an opaque electrode and the light receiving surface has an area covered with the electrode and an incident area not covered with the electrode to permit entry of the light.
(11) The light receiving element of the above-mentioned (8), wherein the ohmic electrode of the other polarity mentioned above is formed via a first conductivity type and low resistance GaN group semiconductor layer, the aforementioned low resistance GaN group semiconductor layer and the light receiving layer are successively formed on a crystal substrate, an upper surface of the low resistance GaN group semiconductor layer is partially exposed, and the ohmic electrode of the other polarity is formed on this exposed surface.
(12) The light receiving element of the above-mentioned (11), wherein the above-mentioned crystal substrate is a sapphire crystal substrate, the above-mentioned low resistance GaN group semiconductor layer is an n+xe2x80x94GaN group semiconductor layer, the above-mentioned light receiving layer is an nxe2x88x92xe2x80x94GaN group semiconductor layer, and the ohmic electrode of one polarity formed on the above-mentioned light receiving surface is a comblike electrode.